One-dimensional Free Electron Laser Research Articles

Statistical theory of a self-seeded free electron laser with noise pedestal growth

We study the impact of a spectral noise pedestal on the statistical properties of a self-seeded free electron laser (FEL). The broad pedestal is assumed to come from self amplified spontaneous emission (SASE) in the second stage of the self-seeding system and is uncorrelated with the narrowband amplified seed. An analytic description is developed based on the statistical theory of a one-dimensional FEL in the high gain linear regime. The theory shows good agreement with experimental data, which show that the presence of a SASE background can have a strong impact on the statistical behavior of self-seeding systems.

Study of the beam tolerance for plasma based ion channel lasers

In this work we use the one-dimensional free electron laser (FEL) model to define necessary parameters of an electron beam to produce self amplified spontaneous emission (SASE) in an ion channel laser (ICL) undulator. Limits and constraints on beam parameters are given, based on theoretical argument and numerical simulations results using the Architect hybrid code.

Wide-Range Third Harmonic Oscillations in Free-Electron Lasers Using the Storage Ring NIJI-IV

We developed third-harmonic free-electron laser (FEL) oscillations that operated from the visible to near-infrared region. Our infrared FEL system uses the storage ring NIJI-IV, which produced the first third-harmonic storage ring FEL oscillations. By measuring the characteristics of the third-harmonic FEL oscillations, we found that behavior of the third-harmonic FEL could be explained by a one-dimensional FEL theory. We demonstrated that the higher harmonic was effective in shortening the FEL wavelength, so it will save electron-beam energy in X-ray FEL oscillators based on a cw energy-recovery linac.

Operation of higher harmonic oscillations in free-electron lasers

We report for the first time the experimental achievement of a seventh-harmonic free-electron laser (FEL) oscillation. The measured FEL gains and average FEL powers for higher harmonics were identical to those calculated by a one-dimensional FEL theory. The measured linewidths of the higher-harmonic FELs were narrower than that of the fundamental FEL owing to the narrower spectral widths of the spontaneous emissions. By applying the higher-harmonic FEL oscillation to a resonator-type FEL with an advanced accelerator, an x-ray FEL oscillator can be realized at lower electron-beam energy.

One-dimensional free-electron laser equations without the slowly varying envelope approximation

A set of one-dimensional equations has been deduced in the time domain from the Maxwell-Lorentz system with the aim of describing the free-electron laser radiation without using the slowly varying envelope approximation (SVEA). These equations are valid even in the case of arbitrarily short electron bunches and of current distributions with ripples on the scale of or shorter than the wavelength. Numerical examples are presented, showing that for long homogeneous bunches the new set of equations gives results in agreement with the SVEA free-electron laser theory and that the use of short or prebunched electron beams leads to a decrease of the emission lethargy. Furthermore, we demonstrate that in all cases in which the backward low frequency wave has negligible effects, these equations can be reduced to a form similar to the usual 1D SVEA equations but with a different definition of the bunching term.

Mode growth and competition in the x-ray free-electron laser oscillator start-up from noise

We describe the radiation properties of an x-ray free-electron laser (FEL) oscillator, beginning with its start-up from noise through saturation. We first decompose the initially chaotic undulator radiation into the growing longitudinal modes of the composite system consisting of the electron beam, the undulator, and the Bragg mirror resonator cavity. Because the radiation initially comprises several modes whose growth rates are comparable, we find that only after many oscillator passes is the output pulse dominantly characterized by the lowest-order Gaussian mode. We verify our analytic results with a novel, reduced one-dimensional FEL code (derived in the text), and with two-dimensional FEL simulations. Understanding the full longitudinal structure during the initial amplification will be critical in assessing the tolerances on the electron beam, undulator, and optical cavity required for robust operation.

Analytical description of free-electron-laser oscillations in a perfectly synchronized optical cavity

We present an analytical description of free-electron-laser (FEL) oscillations in a perfectly synchronized optical cavity by solving the one-dimensional FEL equations. It is shown that the radiation stored in the cavity eventually evolves into an intense few-cycle optical pulse in the high-gain and low-loss regime despite the lethargy effect. The evolution of the leading slope of the optical pulse, which is defined from the front edge toward the primary peak, is found to play an important role in generating the intense few-cycle optical pulse. The phase space evolution of electrons which interact with the leading slope is solved analytically in a perturbation method, leading to an analytical solution for the optical pulse evolution. The peak amplitude and the pulse length at saturation are found to scale with the electron beam density and optical cavity loss. Those scalings agree well with the intense few-cycle pulses recently observed in a high-power FEL.

One-dimensional free-electron laser equations without slowly varying envelope approximation

It is shown how the usual first-order equations describing the one-dimensional free-electron laser dynamics can be obtained without relying on any a priori separation between fast and slow time and space variations in the radiation field (the well-known slowly varying envelope approximation). The exact first-order equations can be reduced to the approximate form and sufficient conditions are given for making this reduction. It is also shown that the start-up of the high-frequency signal is correctly described by the slowly varying approximation only when the electron bunch at t=0 has longitudinal density and/or parallel velocity fluctuations whose spectrum has significant values in the region comprising the wave number of the high-frequency wave.

High gain strong pump free-electron lasers with fast 2π/k w spatial oscillations

In the high gain strong pump regime, the radiation field amplitude and phase can vary substantially over a wiggler period. Also, due to the periodic transverse motion, the longitudinal electron energy suffers large variations over the same distance. Because of these considerations, the spatial average that leads to the classical one-dimensional free-electron laser equations is not applicable here. Thus the rederived equations suitable for the high gain strong pump regime contain 2π/kw periodic terms. Comparisons with the Electron Laser Facility (ELF) experiments of the Lawrence Livermore National Laboratory [Phys. Rev. Lett. 54, 889 (1985); 57, 2172 (1986); Phys. Rev. A 35, 2184 (1987)] on the operational characteristics including the relative phase of the radiation field are made.

Wall shorting of space charge field in free electron lasers

The one-dimensional free electron laser equations with space charge field are extended to include the effect of the conducting waveguide wall. Green function solution of the two-dimensional Poisson equation indicates a wall shorting factor much less than unity which greatly attenuates the space charge field inside the waveguide. Physically, this attenuation is due to the action of the image charges.

Microscopic gain mechanism in the high-gain free electron laser

In this article we present a detailed investigation of the microscopic electron/radiation evolution in a high-gain free electron laser (FEL). Both the steady-state and in particular the superradiant regimes are investigated. Solutions of the one-dimensional FEL evolution equations are represented dynamically by animated computer graphics to produce “movies” of the electrons/radiation evolution. We believe this form of representation of the FEL process provides a clearer, intuitive picture of the underlying mechanisms involved. This has been of particular importance in the understanding of more complex FEL mechanisms such as superradiant phenomena.

Multifrequency effects in an inverse free-electron-laser accelerator.

The electron dynamics in the inverse free-electron-laser (FEL) accelerator is investigated by a one-dimensional FEL simulation code. Changing the gradient of the wiggler-field tapering, the electron bunch length in acceleration buckets and the trapping fraction are found to vary. When an infrared laser of 60 GW is focused to a 280-\ensuremath{\mu}m-diam laser spot, the maximum acceleration rate reaches 100 MeV/m and the bunch length is about 1/14 of the laser wavelength. In multifrequency analysis, an unstable phenomenon is found that is due to the resonance between the synchrotron motion and beat of a broadband laser radiation. The resonance width for the instability is greater than O(0.1) of the synchrotron frequency when the sideband intensity is higher than a few percent of the carrier wave intensity.

Sideband instabilities in free electron lasers

The linear stability of sideband modes for a one-dimensional free electron laser is investigated in detail. The dependence on wiggler taper, slippage between optical pulse and electrons, and trapped electron distribution functions are included in the analysis. Nyquist plots are used to delineate the parameter space in which sideband instabilities occur and approximate analytic expressions for the linear growth rate are derived. In special cases a complete analytic solution is given. Essentially all equilibria are unstable to sideband growth. The linear growth rates agree well with numerical simulations.

One-dimensional simulation of a free electron laser oscillator using a diffraction grating as a cavity mirror

The free electron laser (FEL) oscillator is known to excite parasitic sideband wavelengths in addition to the fundamental wavelengths at high intracavity power. When a tapered wiggler is used, the sidebands reduce extraction efficiency. We studied an FEL cavity with one of the mirrors ruled as a diffraction grating. The diffraction grating deflects the light in the cavity selectively according to the wavelength: the sidebands can be eliminated by deflection to off-axis of the cavity. Because the grating mirror must be mounted with an angle, the laser pulse is stretched after diffraction. The pulse stretching reduces optical intensity. We studied the effect of pulse stretching numerically using a one-dimensional FEL code FELP. (FELP is a one-dimensional free electron laser code written by B.D. McVey, MS-E531, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.) Using this code, the free electron laser performance is calculated and compared with different line densities of grating for the following three wigglers: an untapered wiggler, a 12% wavelength tapered wiggler, and a wiggler tapered in wave number by 30%. The wavelength aperture caused by the grating also is studied as a function of line density.

Nonlinear analyses for optimized short-period-wiggler free-electron laser oscillators

Procedures to determine the parameters of optimized (for maximum efficiency) experimental short-period-wiggler free-electron laser (FEL) oscillators are presented. The calculations are based on a normalized set of nonlinear equations which describe a one-dimensional FEL oscillator. Scaling formulas relating the normalized theoretical parameters to dimensional experimental quantities are obtained. The physics of such realistic effects as waveguide dispersion, wiggler field dependence on magnet gap, beam energy spread, and the frequency tunability of tapered wigglers are considered. Finally, specific examples for both tapered and untapered 300-GHz, 1.0-MW FEL oscillators are calculated and discussed.

Storage Ring Parameters for High Gain FEL

We use one-dimensional free electron laser (FEL) theory to find criteria for choosing electron beam and undulator parameters for operation of a high gain FEL at 400 A. The criteria are moderate electron beam energy (<1 GeV), high peak current (several hundred amperes), small emittance ( about10/sup -8/ m-rad), small relative momentum spread ( about0.001), and narrow undulator gap ( about3 mm). Results of two-dimensional simulations on FEL performance are also presented.